When a clock is used to perform a continuous monitoring process such as metering, changes to the clock must be performed very carefully. The challenge is to quantify the limits by which time can be changed without having an adverse effect on the metering data.
Electric utilities are currently transitioning away from electromechanical meters (typically 2% accuracy class) to solid-state meters (often 0.2% accuracy class). This transition has “raised the bar” to improve the quality of the data delivered by the meter reading system. Furthermore, the transition of the industry from one-way AMR (Advanced Meter Reading) systems to two-way AMI (Advanced Metering Infrastructure) systems has introduced new features such as hourly interval data and meter clock maintenance which “raise the bar” for the need of accurate timekeeping.
The American National Standards Institute (ANSI) “Code for Electricity Metering,” No. C12.1, specifies that meters must have a clock that maintains time with an error no greater than two (2) minutes per week. This corresponds to a maximum allowable slew error rate of 198 μS/S. This however is not the only criteria that must be met. Meters are built to guarantee a certain level of performance in terms of accuracy. In order to maintain such accuracy, meter manufacturers must control a number of unrelated processes inside and outside of the meter.
Meter clocks must be maintained within a prescribed tolerance or the meter function is compromised. The “accuracy class” of many solid state revenue meters today is 0.2%, and the need for 0.1% tolerance has been identified. This presents a challenge for the time synchronization function. Most measurements in the meter are time based. For example, the measurement of energy over a “demand interval” is the observation of usage over a specific period of time. If we assume a continuous, steady flow of energy near the maximum amount allowed by the meter, even “small” changes to time can affect data in a corresponding manner and cause the meter to fail to measure data accurately.
Demand is commonly measured over a 15 minute interval. Likewise, a common practice has been to broadcast a time synchronization message (i.e., a time sync) every 15 minutes to the communication modules. This conceivably causes every demand interval calculated to be affected by the phenomenon. With a system running one (1) minute interval analysis on a 0.1% metrology system, this implies that a change of merely 60 mS could disrupt the quality of the data.